Protective coating method of pervoskite structure for SOFC interconnection

ABSTRACT

A protective coating is formed on a stainless interconnecting plate used in solid oxide fuel cell (SOFC). With the protective coating, a contact resistance of the plate is effectively lowered. Anode and cathode of SOFC are also prevented from being poisoned by chromium diffusion from the plate. Therefore, after a long time of use under a high temperature, a degradation rate for power generating of SOFC is reduced; and, thus, a working hour is prolonged. Hence, the SOFC can be mass-produced and large-scaled.

FIELD OF THE INVENTION

The present invention relates to a method for protective coating; moreparticularly, relates to forming a protective coating of a pervoskitestructure on a stainless interconnecting plate used in solid oxide fuelcell (SOFC).

DESCRIPTION OF THE RELATED ARTS

A few methods for a protective coating of a pervoskite structure on astainless interconnecting plate include a radio frequency (RF) plasmamagnetron sputtering, a plasma spray and sol-gel, and an ion beamsputtering to paste a protective coating of La_(0.67)Sr_(0.33)MnO₃ (LSM)to obtain the pervoskite structure through annealing.

But these methods have some disadvantages. Concerning the RF plasmamagnetron sputtering, the RF power supply used is expansive and itsdeposition rate is slower than that of a pulsed DC power supply.Concerning the sol-gel, it is hard to control its crystallization andits adhesion of coating film is not good; in addition, its structure isnot close-grain ed, thus it is not a good protective coating under ahigh temperature. Concerning the plasma spray, the plasma sprayparticles are bigand so the thin film obtained has a porous structure,which does not suit to be a protective coating.

A prior art of U.S. Pat. No. 5,426,003, “Method of forming a plasmasprayed interconnection layer on an electrode of an electrochemicalcell”, fabricates an interconnecting layer through a plasma spray. Butthe thin film obtained is not close-grained, the post-processing is noteasy and the cost is high too. Another prior art of U.S. Pat. No.5,609,921, “Suspension plasma spray”. A protective coating is depositedthrough a plasma spray. Because the thin film obtained through theplasma spray is rapidly cooled down, some defects may happen to the thinfilm and thus the protective coating may fail under a high temperature.

A ceramic interconnection can be obtained using the above prior arts,but it is expansive. On the contrary, a stainless substrate is cheap andis easily processed. But an interface resistance between an anode and acathode increases after a long time of operation under a hightemperature; and its anode and cathode may be poisoned by chromium.Therefore a protective coating is required to block the chromium fromdiffusing to the anode and the cathode. Yet the above prior art is noteither close-grained or tightly adhered. Hence, the prior arts do notfulfill users' requests on actual use.

SUMMARY OF THE INVENTION

The main purpose of the present invention is to preparing a stainlessinterconnecting plate having a protective coating of pervoskitestructure to be used in SOFC to effectively reduce a contact resistanceof the stainless interconnecting plate and to prevent anode and cathodeof SOFC from being poisoned by the diffusion of chromium from thestainless interconnecting plate.

To achieve the above purpose, the present invention is a protectivecoating method of a pervoskite structure for SOFC interconnection,comprising steps of: (a) deposing a stainless interconnecting plate on aholder substrate in a vacuum chamber and pumping the vacuum chamber to avacuity through a pumping device; (b) accessing a gas into the vacuumchamber to maintain a gas pressure and processing a DC discharge with apulsed DC power supply to obtain a plasma; and (c) bombarding apervoskite structure target on a surface of the target by reactive ionsin the plasma through a field control to sputter the pervoskitestructure on the stainless interconnecting plate to obtain a protectivecoating and processing the stainless interconnecting plate throughannealing to obtain the stainless interconnecting plate having theprotective coating of the pervoskite structure. Accordingly, a novelprotective coating method of a pervoskite structure for SOFCinterconnection is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood from the followingdetaiIed description of the preferred embodiment according to thepresent invention, taken inconjunction with the accompanying drawings,in which

FIG. 1 is the view showing the device used according to the presentinvention;

FIG. 2 is the view showing the flow chart of the present invention;

FIG. 3 is the view showing the X-ray powder diffraction analysis;

FIG. 4 is the view showing the protective coating by the electronmicroscope; and

FIG. 5 is the view showing the ASR.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The following description of the preferred embodiment is provided tounderstand the features and the structures of the present invention.

Please refer to FIG. 1 and FIG. 2, which are views showing a device usedand a flow chart of the present invention. As shown in the figures, thepresent invention is a protective coating method of a pervoskitestructure for SOFC interconnection. A device used according to thepresent invention is a vacuum chamber 11, comprising a holder substrate111, a cathode 112, at least one anode 113, a shielding shell 114 and avalve 115. The vacuum chamber 11 connects to a pumping device 12, apulsed DC power supply 13 and a bias 14, where the pulsed DC powersupply 13 is connected with the cathode 112 and the anode 113 is theshell of the vacuum chamber 11.

The present invention prepares a protective coating of a pervoskitestructure on a stainless inter-connecting plate through the followingsteps:

(a) Deposing a stainless inter-connecting plate on a holder substrate ina vacuum chamber having a vacuity 21: A stainless interconnecting plate1111 is deposed on a holder substrate 111 in a vacuum chamber 11 and thevacuum chamber 11 obtains a vacuity by exhausting air through a pumpingdevice. Therein, the stainless inter-connecting plate 1111 is made of aFe(iron)-base alloy, a Cr(chromium)-base alloy, a Ni(nickel)-base alloyor an alloy made of any combination of the above alloys. The vacuity isbelow 10⁻⁴ torr. The holder substrate 111 is further equipped with aheating rotator to heat and rotate the holder substrate 111. The cathode112 is cooled down with a cooling water to absorb heat from thepervoskite structure target 15 on plasma discharging. The shieldingshell 114 preserves plasma on a surface of the pervoskite structuretarget 15 to keep from wasting. The holder substrate 111 has a potentialfurther added by a bias 14. The bias 14 has a voltage located between−150 volts (V) and 0V to enhance the speed and efficiency of thesputtering and forming of the protective coating. The potential of theholder substrate 111 and that of the anode 113 are ground potentials.And the molecular formula of the pervoskite structure target 15 is ABO₃,where the ‘A’ is Ln_(x)E_(1-x); the Ln is a rare earth element; the E isan alkaline—earth metal; the x is a value greater than 0.1 and smallerthan 0.9; and the B is a transition metal.

(b) Processing a DC discharge to obtain a plasma 22: After the vacuumchamber 11 obtains the default vacuity, a gas is accessed, which isargon (Ar), krypton (Kr), oxygen (O₂) or a gas mixed of any combinationof the above gases. A valve 115 is used to remain the vacuum chamber 11in a pressure between 0.001 torr and 0.1 torr. The pulsed DC powersupply 13 is processed with a DC discharge to obtain a plasma from thegas, where the DC discharge has a volt lower than 1000V; and the pulsedDC power supply 13 has a frequency between 0 and 350 kilo hertz (KHz).The power and time used is decided according to the state on fabricatingthe protective coating of a pervoskite structure.

(c) Sputtering a pervoskite structure on the stainless inter-connectingplate to form a protective coating before annealing 23: Reactive ionsobtained from the plasma and the gas bombard the pervoskite structuretarget 15 with a field control to sputter the pervoskite structure onthe stainless interconnecting plate 1111 for forming a protectivecoating. Then the stainless interconnecting plate 1111 having theprotective coating is put in a furnace for processing an annealing tofurther obtain a stainless interconnecting plate 1111 having theprotective coating of the pervoskite structure, where the temperaturefor the annealing is higher than 600 Celsius degrees (° C.).

Thus, a novel protective coating method of a pervoskite structure forSOFC interconnection is obtained.

Take fabricating a protective coating of a pervoskite structure for astainless interconnecting plate of Crofer22, for example. Thefabricating method comprises the following steps:

(a) A stainless interconnecting plate of Crofer22 having an area of10×10 mm (millimeter) and a thickness of 5 mm is put on a holdersubstrate 111 in the vacuum chamber 11. Then the valve 115 is opened toexhaust gas by the pumping device to obtain a vacuity of 5×10⁻⁵ torr.

(b) A gas is accessed, which is Ar with a flow rate of 60 standard cubiccentimeters per minute (sccm). The pressure in the vacuum chamber 11 iskept at 0.02 torr by using the valve 115 The cathode 112 is cooled downwith a cooling water. The potentials of the holder substrate 111 is aground potential. The distance 17 between the holder substrate 111 andthe pervoskite structure target 15 is about 5 centimeters (cm). Theshell of the vacuum chamber 11 is the anode 113 with a ground potential.Then the pulsed DC power supply 13 is turned on for a DC dischargebetween two electrodes to produce a plasma through reacting with thegas. There in, the DC discharge has a voltage of 200V; and the pulsed DCpower supply has a frequency of 350KHz together with a power of 100walts run for 2 hours.

(c) Reactive gas ions in the plasma bombard a pervoskite structuretarget 15 under a field control to sputter a pervoskite structure(La_(0.67)Sr_(0.33)MnO₃, LSM) on the stainless interconnecting plate toform a protective coating. Then the stainless interconnecting plate 1111having the protective coating is processed with four periods of one hourof annealing at 600° C., 700° C., 800° C. and 900° C. separately.

Please refer to FIG. 3 to FIG. 5, which are views showing an X-raypowder diffraction analysis, a protective coating by the electronmicroscope and an area specific resistance (ASR). As shown in FIG. 3,there are a first diffraction curve 31, a second diffraction curve 32, athird diffraction curve 33 and a fourth diffraction curve 34, where thefirst diffraction curve 31 is the diffraction curve obtained from theannealing at 600° C.; the se con d diffraction curve 32, at 700° C.; thethird diffraction curve 33, at 800° C.; and the fourth diffraction curve34, at 900° C. From the first diffraction curve 31, the seconddiffraction curve 32, the third diffraction curve 33 and the fourthdiffraction curve 32, it is known that, when the annealing temperatureis higher than 700° C., a peak 321, 331, 341 is obtained for theprotective coating of the pervoskite structure on processing one hour ofan annealing.

As a result, a protective coating of the pervoskite structure processedwith one hour of annealing at 700° C. is obtained; and, as shown in FIG.4, its cross-section 41 is close-grained. Then the protective coating ofthe pervoskite structure is measured with its are a specific resistance(ASR). As shown in FIG. 5, by measuring at 750° C. for hundreds ofhours, a diffraction curve 51 is obtained, whose resistance is about0.0395 Ωcm², smaller than the least requirement of 1 Ωcm² for a solidoxide fuel cell (SOFC).

To sum up, the present invention is a protective coating method of apervoskite structure for SOFC interconnection, where a close-grainedprotective coating of a pervoskite structure is formed after anannealing to a stainless interconnecting plate sputtered with aprotective coating; and, by doing so, easy-fabricated and cheapstainless steel can be used as an interconnecting plate for SOFC used ina high temperature.

The preferred embodiment herein disclosed is not intended tounnecessarily limit the scope of the invention. Therefore, simplemodifications or variations belonging to the equivalent of the scope ofthe claims and the instructions disclosed herein for a patent are allwithin the scope of the present invention.

1. A protective coating method of a pervoskite structure for SOFCinterconnection, comprising: (a) deposing a stainless inter-connectingplate on a holder substrate in a vacuum chamber and pumping said vacuumchamber through a pumping device to obtain a vacuity; (b) accessing agas into said vacuum chamber to maintain a gas pressure and processing aDC discharge with a pulsed DC power supply to obtain a plasma; and (c)bombarding a pervoskite structure target on a surface of said target byreactive ions in said plasma with a field control to sputter saidpervoskite structure on said stainless interconnecting plate to obtain aprotective coating and processing annealing to said stainlessinterconnecting plate to obtain said stainless inter-connecting platehaving said protective coating of said pervoskite structure.
 2. Themethod according to claim
 1. wherein said stainless inter-connectingplate is made of a material selected from a group consisting of a Fe(iron)-base alloy, a Cr(chromium)-base alloy and a Ni(nickel)-basealloy.
 3. The method according to claim
 1. wherein said pervoskitestructure has a molecular formula of ABO₃; wherein said A isLn_(x)E_(1-x), said Ln is a rare earth element, said E is analkaline—earth metal, and said X is greater than 0.1 and is smaller than0.9; and wherein said B is a transition metal.
 4. The method accordingto claim 1 wherein said gas is selected from a group consisting of argon(Ar), krypton (Kr) and oxygen (O₂).
 5. The method according to claim 1.wherein said DC discharge has a voltage smaller than 1000 volts (V). 6.The method according to claim
 1. wherein said gas pressure is locatedbetween 0.01 torr and 0.1 torr.
 7. The method according to claim 1wherein said pulsed DC power supply has a frequency between 0 hertz (Hz)and 1 mega Hz (MHz).
 8. The method according to claim 1 wherein saidpulsed DC power supply is connected with a cathode.
 9. The methodaccording to claim 1 wherein said holder substrate has a potentialfurther added by a bias; and wherein said bias has a voltage locatedbetween −150V and 0V.
 10. The method according to claim 1 wherein saidholder substrate has a ground potential.
 11. The method according toclaim 1 wherein said annealing has a temperature higher than 600 Celsiusdegrees (° C.).